1. Field of the Invention
The present invention relates to a wafer transfer apparatus, and more particularly, it relates to an apparatus for transferring a plurality of wafers from one wafer carrier to another in a fabricating process of semiconductor devices.
2. Description of the Prior Art
Generally, semiconductor devices are fabricated on a wafer, and wafer carriers, which will be simply called "carriers" hereinafter, are used for carrying or stocking the wafers in the fabricating process of the semiconductor devices. Since the usage of the carriers depends on a respective process of fabricating the semiconductor devices, it is necessary to transfer the wafers from one carrier used in a former process to another carrier in a next process. Therefore, it is important that the apparatus for transferring the wafers, which will be called "a wafer transfer apparatus" hereinafter, operates in a stable manner because the wafers are very delicate.
The carrier usually has an external shape of an approximately rectangular prism as shown in FIGS. 1(a) and 1(b), and one side (a first side) of the carrier is open for providing a first opening for receiving or taking out the wafers W from the carrier. A plurality of wafers W, twenty five wafers for example, can be held in the carrier, arranged in parallel leaving a space of 4.76 mm between adjacent wafers.
There are two types of carriers: a process type and a box type. FIG. 1(a) shows a typical process type carrier which is to be used in a wafer process such as a process of washing, photo lithography, etching, diffusion and heat treatment. The process type carrier is made of, for example, quartz glass or plastics. The process carrier has another opening, which will be called "a second opening" hereinafter, in the side opposite to the first opening. The second opening is for moving the wafers back and forth from the exterior of the process type carrier. FIG. 1(b) shows a typical box type carrier for transporting or stocking the wafers. The box type carrier is made of, for example, plastic and only has the first opening because the less opening the better in order to keep out dust during the transportation or the stocking.
A wafer transfer apparatus is for transferring the wafers W from one wafer carrier to another. In FIG. 2, a typical, prior art wafer transfer apparatus and two box type carriers C1 and C2 mounted on a turn table 1 of the wafer transfer apparatus are illustrated. A schematic side view for the prior art wafer transfer apparatus and a schematic sectional side view are depicted for the box type carriers C1 and C2. FIG. 2 shows that the wafers W stocked in the box type carrier C1 which are to be transferred to the box type carrier C2, using a wafer transfer unit 2 of the wafer transfer apparatus. In FIG. 2, the turn table 1 of the wafer transfer apparatus is turned on an axis 1a thus turning the box type carriers C1 and C2 simultaneously. When the wafers W in the box type carrier C1 are transferred to the box type carrier C2, the transfer is carried out in accordance with the following steps of: (1) turning the box type carrier C1 so that the opening (the first opening) of the carrier C1 faces the wafer transfer unit 2 (FIG. 2 shows this state); (2) inserting arms 2a of the wafer transfer unit 2 into the spaces between the wafers W by shifting the wafer transfer unit 2 in a direction B; (3) chucking a wafer W placed above each arm 2a by using a chuck 2b equipped on each arm 2a; (4) shifting the wafer transfer unit 2 back in a direction A thereby removing the wafers W from the carrier C1; (5) turning the turn table 1 so that the opening (the first opening) of the carrier C2 faces the wafer transfer unit 2; (6) inserting the wafers W into the carrier C2 by shifting the wafer transfer unit 2 in the direction B; (7) placing the wafers W in the carrier C2 by releasing the chuck 2b; and (8) shifting the wafer transfer unit 2 back in the direction A.
However, in the above steps, particularly in the above step (2), since the space between the wafers in the carrier C1 or C2 is very narrow as described before, the arm 2a must be very thin and, therefore, the arm 2a is mechanically very weak. Accordingly, in the wafer transfer apparatus of the prior art, it is very hard to correctly position the end of arms 2a between the wafers in the carrier C1 and the insertion of the arms 2a becomes unstable. This is a first problem in the wafer transfer apparatus of the prior art.
The above first problem may be solved if the arms 2a are not used, and this may be achieved by applying an idea of the Japanese Laid-Open patent No. 50-34481 to Jun Suzuki, which will be called as "Suzuki" hereinafter. According to "Suzuki", wafers can be transferred from one carrier to another without using arms. Namely, wafers are transferred using the weight of the wafers themselves. Each of two carriers used in "Suzuki" is a process type having a first opening through which wafers are transferred and a second opening through which a supporter corresponding to each carrier is inserted into the carrier. The first opening and the second opening of each carrier are located on sides opposite to each other. The transfer of the wafers is carried out in accordance with the following steps:
(1) A first carrier, in which wafers to be transferred are stocked, is set so that a side of the first opening of the first carrier is directed upward. The wafers are supported by a first supporter which corresponds to the first carrier and is inserted into the first carrier through the second opening of the first carrier so that the wafers are supported on the first supporter.
(2) A second carrier, to which the wafers will be transferred, is put on the first carrier so that the first opening of the second carrier is directed downward and joined to the first opening of the first carrier. Then, a second supporter corresponding to the second carrier, supports the wafers so as to touch the upper side of the wafers.
(3) The first and second carriers which are joined together are turned upside down and the wafers in the first carrier are transferred to the second carrier using their own weight by moving the second supporter of the second carrier downward. At that time, the first supporter of the first carrier is also moved down lightly touching the upper side of the wafers.
(4) The first carrier is separated from the second carrier, and the transfer of the wafers is, thus, completed.
In the above explanation, the second supporter is necessary for preventing the wafers from rolling down into the second carrier during the transference of the wafers, and the first supporter works so that the wafers can be transferred smoothly irregular transferring motion is avoided. As can be seen from the above explanation of the steps, the second openings of the first and the second carriers are necessary to allow the first and the second supporters to operate. Thus, "Suzuki" can be used only when the carriers are the process type, in other words, the box type carriers cannot be used in "Suzuki" because the box type carrier has only the first opening. This is a second problem.